1. Field of the Invention
The present invention relates to an optical fiber transmission line and an optical fiber communication system using the optical fiber transmission line.
2. Description of the Related Art
In recent years, a transmission system capable of performing direct amplification of light has been studied owing to the advent of an erbium doped optical fiber amplifier. In particular, the erbium doped optical fiber amplifier has a wide gain wavelength band and therefore allows wavelength division multiplexing (WDM) transmission and collective amplification and relay of each wavelength. However, the entry of a wavelength multiplex signal having a high light power into an optical fiber causes a new problem such that a nonlinear optical effect in the optical fiber becomes remarkable.
An optical fiber now in use is formed of a silica-based material, which is essentially very small in nonlinearity. However, since a light wave is confined in a microscopic region of about 10 .mu.m in diameter, a power density becomes very high, and various nonlinear interactions may arise remarkably because of a very large length of interaction between the light wave and the material. Accordingly, the nonlinear optical effect causes a deterioration of transmission characteristics in WDM transmission. The nonlinear optical effect of an optical fiber having an adverse effect on WDM includes stimulated Brillouin scattering, mutual phase modulation, Raman scattering, and four wave (photon) mixing.
According to the literature "IEEE J. Lightwave Technol., vol. 6, no. 11, pp. 1750-1769", the four wave mixing (FWM) of the above-mentioned nonlinear optical effect gives the severest conditions to the design of a communication system. That is, light frequency mixing between signal light waves due to FWM causes generation of new FWM waves, which act as a crosstalk with the original signal light waves to deteriorate the transmission characteristics. The generation efficiency of FWM is decided by a quantity .DELTA..beta. of phase mismatch between light waves, and .DELTA..beta. is dependent upon the wavelength space between light waves and the dispersion of an optical fiber. Therefore, in the case of using as the transmission line a dispersion shifted fiber such that a zero dispersion region of the fiber is shifted to a 1.5 .mu.m band where a transmission loss is minimized, the influence of the FWM becomes remarkable.
As an example, FIG. 10 shows a difference in the influence of FWM between two kinds of optical fibers as shown in the literature "IEEE J. Lightwave Technol., vol. 10, no. 3, pp. 361-366". In FIG. 10, the solid line shows a usual fiber, and the broken line shows a dispersion shifted fiber. As the dispersion shifted fiber is largely affected by crosstalk, it is necessary to reduce an input power into the optical fiber, so that the transmission characteristics are largely limited. The results of study mentioned in the above literature are those obtained with the assumption that the characteristics of the optical fiber are ideal, i.e., a zero dispersion wavelength is constant in the longitudinal direction of the fiber. Actually, however, it is considered that there is a change in dispersion value in the longitudinal direction of the optical fiber due to variations in manufacturing conditions of the optical fiber.
Examples of measurement of the distribution of generation efficiency of FWM in an actual dispersion shifted fiber are shown in FIGS. 11 and 12. FIG. 11 shows the relation between a signal light wavelength and an FWM generation efficiency in the case where the fiber length is 1.1 km, and FIG. 12 shows the same relation in the case where the fiber length is 23 km. In each of FIGS. 11 and 12, the broken line shows a calculated value and the solid line shows an experimental value. When the optical fiber is short as shown in FIG. 11, the calculated value and the experimental value are in good agreement with each other, and the generation of FWM is observed at a specific wavelength. To the contrary, when the optical fiber is long as shown in FIG. 12, the steep peak predicted by calculation is not measured in actual, but the FWM wave measured is distributed in a wide range of wavelength. It is understood that this result is due to the variations in dispersion value in the longitudinal direction of the optical fiber.
FIG. 13 shows the result of measurement of variations in zero dispersion wavelength (wavelength where the dispersion value becomes zero) in the longitudinal direction of the optical fiber. This result is one obtained by using the optical fiber corresponding to that shown in FIG. 12, and it is understood that a variation of about 3.5 nm is observed. A maximum inclination is 1.1 nm/km. If the variations in zero dispersion wavelength in the longitudinal direction of the optical fiber are always present, the generation efficiency of FWM is widely distributed, but the generation efficiency of FWM at a specific wavelength is reduces.
As an example, the literature "Electronic Information and Communication Society, Autumn Great Meeting B-660, 1992" has proposed a method of constructing a transmission line by alternately connecting a fiber having a positive dispersion value and a fiber having a negative dispersion value and making the total quantity of dispersion zero as shown in FIG. 14. This literature has reported that such a construction of the transmission line has improved the transmission characteristics. A primary object of the transmission line shown in this literature is to reduce the crosstalk due to the interaction between noise and signal light from a low-power light amplifier in the case of transmitting a one-wave signal over a long distance.
In general, a noise component is enough smaller than a signal light power, and the generation of FWM between noise and signal light becomes a problem in the transmission over the distances of hundreds of kilometers. Accordingly, it is sufficient to alternate the positive dispersion value and the negative dispersion value at the intervals of tens of kilometers, and it is unnecessary to consider the distribution of the zero dispersion wavelength in each section in the longitudinal direction of the fiber. For example, there is no harm in making completely uniform the zero dispersion wavelength in each section of the fiber.
In contrast, it is considered that a WDM transmission system may generate FWM between high-power signal lights. If a zero dispersion wavelength and a signal light wavelength are very close to each other even over a short distance, the generation efficiency of FWM is rapidly increased. For example, if the zero dispersion wavelength is constant and the signal light wavelength is the same as or close to this even over a short distance of 1 to 2 km, a crosstalk having an influence on the transmission characteristics is generated. Accordingly, the above-mentioned method is not effective in suppressing the crosstalk having an influence on the transmission characteristics.
Further, it is necessary to select a specific fiber having suitable characteristics from many fibers, which causes a problem in practical use. Further, according to the result of measurement on the actual fibers, it cannot be considered that the zero dispersion wavelength in each span is uniform, and it is essentially difficult to construct the transmission line by combining the fibers having positive or negtive dispersion values and making the total dispersion amount zero. The dispersion shifted fiber at present has variations difficult to control due to manufacturing conditions. For example, the literature "IEEE j. Lightwave Technol., vol. 8, no. 10, pp. 1476-1481" has reported 0.03 .mu.m as a measurement value of variations in core diameter.
Further, it is anticipated that a difference .DELTA.n in specific refractive index between a core and a clad is also varied according to manufacturing conditions. This difference causes variations in zero dispersion wavelength in the fiber. In the case of WDM transmission by the use of such a wavelength, the amount of crosstalk due to FWM becomes smaller than that in the estimation that the zero dispersion wavelength in the fiber is theoretically uniform in the longitudinal direction of the fiber. However, if the distribution of variations in zero dispersion wavelength follows a normal distribution, there is a possibility of production of fibers excluding the variations in the longitudinal direction with a given probability. In the case where a wavelength multiplex signal is transmitted through a transmission line constructed by randomly selecting these fibers, there is a possibility that FWM may be largely generated.